larry00

I would check your battery as there is a point when the battery will turn over the engine alright but the ECM will not have the 13 required volts to fire the engine till the alternator spins up a bit more.
Try another battery or if you have access to a jump start 200 amp quick charge battery charger stick it on and start when hot.

A new OPTI is no guarantee its good ! Especially aftermarket OPTIS !
A lot of guys [ me included ] have found this out the hard way.

Check that your starter has a heat shield because an overheated starter will drag and requires more energy to turn the engine over.

94vettelover2

Like what was previously stated have a battery charger or battery jumper pack & hook upto battery & see if it fires right away.To me it seems like your opti might be the problem or the computer getting warm/hot causing a loose or poor connection.Clean all your battery connections & see if the battery or previous battery was leaking thus damaging wires beneath it.There is a wiring junction block behind battery-make sure those connections are clean

powerpigz-51

Clean and check all electrical grounds....if this has not been done already.

maranello_man

All,

Thanks for the suggestions. I'm always amazed at how helpful people are on the forums. I think that's part of what's great about America, that total strangers still jump in to help each other. This thing has been nagging me for 8 months, and it's time to make it go away.

I;m going to try a few things tonight and I'll update everyone tomorrow.

maranello_man

When the engine is cold and fires up very quickly, there is no check engine light, and the AC blows cold. When the engine takes a long time cranking before it starts, I get a check engine light, and the AC clutch is disabled. This happens everytime without fail....

In the middle of writing this, I discovered in the factory service manual on page 6E3-C10-4, that "The PCM will not activate the A/C clutch with a stored DTC." So, what appears to be happening is that the engine, for whatever reason, does not fire in a certain length of time. Once the engine has turned over a certain number of cycles, Code 36 is set indicating that there is no high res pulse. The high res pulse is not needed to start, but simply cranking that long without generates a code and there you go. No AC.

I tried cooling down the PCM when it was hot, and no change. I swapped out the PCM (I bought a while ago, when I was determined the problem could be nothing else), and no change.

I don't know where all the grounds are, but the ones I found appear to be intact. The location description in the FSM isn't great. Please let me know if anyone knows where these are.

I don't believe battery is the problem as the battery is new, the terminals are clean, and the voltages I get at the PCM seem acceptable. I don't have anything but a trickle charger, and no battery pack.

BTW - Thanks for offer of the PCM swap! That was very kind. Now, if you meant the whole car, maybe we should talk!

powerpigz-51

I am curious as to whether it will start right away in it's "hot" long crank condition if the computer is reset, by pulling the battery ground for 30 seconds. This would help determine if it is a heat soak type fault or not......

maranello_man

Good idea, doesn't seem to change anything.

gbrtng

I'm just babbling here because I'm an L98 and LT5 guy but ...
It sounds like an engine sensor is acting up - since I don't know what sensors the LT1 PCM uses as inputs for its start and run parameters, The best way to continue is with a TECH 1 while watching the sensor data both when cold and when hot.
Maybe somebody who has access to the PCM software logic could jump in here too.

And, the oil pressure sensor is probably NOT causing the long crank interval - at least it does not on the earlier L98.

maranello_man

I think you're exactly right. At this point, I see three possibilities:

1. Some other input to the PCM (one of the sensors) causes the PCM to not supply the ignition pulse for a given length of time. I'm debating hacking into the low res signal from the optispark, and sending out my own pulse to the ICM. I have the hardware and software to do that. Then I would know that it truly is the PCM, that for whatever reason, is not sending the pulse and that the optispark pulse is just fine.
2. The new optispark I put in has some kind of phase shift or other problem (when hot) that knocks the PCM off it's guard (sends it something it doesn't expect) and causes it to not output the ignition pulse for a certain number of cycles. Although I can see a beautiful waveform on the low res signal from the optispark, I don't have a basis for comparison for the phase shift.
3. The wire from the PCM to the ICM is being pulled down somewhere (not to ground, I can and have measured that).

So yes, I can see the inputs to the PCM, but I can't get the info from the PCM to know their values without a Tech1. So, I either pay someone with a Tech1 to go in there, or perform the operation mentioned in #1 above.

powerpigz-51

I am looking at my service manual from Howell Engine Developments. First time I have looked at it, very detailed. According to them, you need the Low res signal (CKT453), and "System ground" (CKT631) and IC (CKT423) for the car to run. No other sensor input required, as far as I can tell. CKT631 provided ground for the opti to generate both low and high reference signals. Since you have these signals, this circuit seems to be working. I would find the large ground that connects the ECM, and make sure it is good, or even add a new one just for grins. If this circuit becomes open, it will not throw a code and the engine will not run. Any other problems with the high or low res, or IC circuits will throw a code if malfunctioning (as in not receiving a signal). I will also throw this out there....when changing the PCM, did you have to install the old MEMCAL?

maranello_man

powerpigz-51,

All four grounds to the PCM probe at .5 ohms, which seems about right. Probing the block to the battery I get about .3 ohms, so another .2 in the connection doesn't surprise me.

I'm not sure if all the grounds are electrically connected inside the PCM or not (so if a connection point is bad in one of the four connectors, the others work out of redundancy).

As far as MEMCAL, I guess that's the PROM?

The PCM I bought was one that had been reprogrammed with an aftermarket image. I haven't swapped out the chips.

Al Borman

Have you considered the possability that your Opti is bad, and that the dual optical sensor is exhibiting heat sensitivity ? Even though the engine takes 6 or so seconds to fire, The Opti should NOT be throwing a Code 36. Those 360 radial high res. pulses should be going from the Opti optical sensor to the PCM whether the engine is running or just turning over.

Do you know how many miles are on current Opti ?

powerpigz-51

OK....when you get the "Check Engine" light when it won't start....is it steady or flashing......Oh I see...flashing code 36....I have a diagnostic flow chart in front of me....bear with me

maranello_man

There are about 50 miles on the current opti. I know because I changed it, along with the PCM, and the ICM, and the wire harness... yuck.

I am also curious why the high res pulse is not there. Code 36 sets when "40 low resolution pulses occur before any high resolution pulses are detected."

I ALWAYS get high res pulses when the engine goes above a certain RPM. I don't know why this happens and it doesn't seem like correct behavior to me either. The opti was purchased online from "Rock Auto," not from GM so yes, it could be suspect.

Tomorrow I'm going to buy a can of electric freeze spray and cool down some of the connectors. The car doesn't have to be at 190 to not work. It can sit out in the hot Texas sun all day without running and it will have the hard start condition. This seems more like a connector issue from one of the many devices that is a PCM input.

There were several issues present when I changed the opti. This hard start may have started when I changed it, so it's certainly still on the list. The only thing that keeps me from looking into it further (besides the fact that it's expensive and hard to change) is that I get a great low res signal consistently which all I "should" need.

We'll see what tomorrow brings. Thanks for hanging in there.

maranello_man

If the car takes too long to start, the check engine light comes on solid about 1 full second AFTER the engine is running. If the engine starts in 2 seconds or less (because it was in the air conditioned garage) then no check engine light comes on.

powerpigz-51

"High Resolution Signal reference....this provides the ECM with detailed reference signals and crankshaft position information. If this circuit becomes open or grounded a DTC 36 will set and the engine will still run"......I wonder if the time it takes to set the code (40 low resolution pulses) is the time delay in the hot start.

powerpigz-51

ok....can you get the computer to flash a code 12 when you ground the diagnostic terminal? (non running)

maranello_man

Yes, using the paper clip method I get a code C12.

If the engine cranks at 600 RPM (my assumption), that's 10 rev/sec. If 6 seconds go by before it starts, that's 60 revs to start. 8 pulses per revolution of cam is 4 pulses per revolution of crank. That's 240 low res pulses to start.

The engine would have to start in less than 1 second at 600 RPM cranking if the high res pulse didn't show up until the engine was running in order to not set a code 36. I think the high res pulse not showing up until the engine is running is incorrect operation, but not necessarily related to the no start...

powerpigz-51

So even though the engine will run without the hi res signal, it will not run until the code has been set, which is 40 low res pulses without a high res pulse..... (thinking out loud)

maranello_man

Yeah, in the hot start video you can see that the high res pulse starts after the ICM pulse has already started AND increased in frequency (engine speed increased). I don't have a good recording of what happens when the car is cold. It would be interesting to see if the high res pulse starts up immediately when the car is cold and even starts before the ICM pulse.